Solar Power Game-Changer: "Near Perfect" Absorption of Sunlight, From All Angles

A new antireflective coating developed
by researchers at Rensselaer could help to overcome two
major hurdles blocking the progress and wider use of
solar power. The nanoengineered coating, pictured here,
boosts the amount of sunlight captured by solar panels
and allows those panels to absorb the entire spectrum of
sunlight from any angle, regardless of the sun’s position
in the sky.

Credit: Rensselaer/Shawn Lin

Researchers at Rensselaer Polytechnic Institute have
discovered and demonstrated a new method for overcoming two
major hurdles facing solar energy. By developing a new
antireflective coating that boosts the amount of sunlight
captured by solar panels and allows those panels to absorb the
entire solar spectrum from nearly any angle, the research team
has moved academia and industry closer to realizing
high-efficiency, cost-effective solar power.

“To get maximum efficiency when converting solar power into
electricity, you want a solar panel that can absorb nearly
every single photon of light, regardless of the sun’s position
in the sky,” said Shawn-Yu Lin, professor of physics at
Rensselaer and a member of the university’s Future Chips
Constellation, who led the research project. “Our new
antireflective coating makes this possible.”

Results of the year-long project are explained in the paper
“Realization of a Near Perfect Antireflection Coating for
Silicon Solar Energy,” published this week by the journal
Optics Letters.

An untreated silicon solar cell only absorbs 67.4 percent of
sunlight shone upon it — meaning that nearly one-third of that
sunlight is reflected away and thus unharvestable. From an
economic and efficiency perspective, this unharvested light is
wasted potential and a major barrier hampering the
proliferation and widespread adoption of solar power.

After a silicon surface was treated with Lin’s new
nanoengineered reflective coating, however, the material
absorbed 96.21 percent of sunlight shone upon it — meaning that
only 3.79 percent of the sunlight was reflected and
unharvested. This huge gain in absorption was consistent across
the entire spectrum of sunlight, from UV to visible light and
infrared, and moves solar power a significant step forward
toward economic viability.

Lin’s new coating also successfully tackles the tricky
challenge of angles.

Most surfaces and coatings are designed to absorb light —
i.e., be antireflective — and transmit light — i.e., allow the
light to pass through it — from a specific range of angles.
Eyeglass lenses, for example, will absorb and transmit quite a
bit of light from a light source directly in front of them, but
those same lenses would absorb and transmit considerably less
light if the light source were off to the side or on the
wearer’s periphery.

This same is true of conventional solar panels, which is why
some industrial solar arrays are mechanized to slowly move
throughout the day so their panels are perfectly aligned with
the sun’s position in the sky. Without this automated movement,
the panels would not be optimally positioned and would
therefore absorb less sunlight. The tradeoff for this increased
efficiency, however, is the energy needed to power the
automation system, the cost of upkeeping this system, and the
possibility of errors or misalignment.

Lin’s discovery could antiquate these automated solar
arrays, as his antireflective coating absorbs sunlight evenly
and equally from all angles. This means that a stationary solar
panel treated with the coating would absorb 96.21 percent of
sunlight no matter the position of the sun in the sky. So along
with significantly better absorption of sunlight, Lin’s
discovery could also enable a new generation of stationary,
more cost-efficient solar arrays.

“At the beginning of the project, we asked ‘would it be
possible to create a single antireflective structure that can
work from all angles?’ Then we attacked the problem from a
fundamental perspective, tested and fine-tuned our theory, and
created a working device,” Lin said. Rensselaer physics
graduate student Mei-Ling Kuo played a key role in the
investigations.

Typical antireflective coatings are engineered to transmit
light of one particular wavelength. Lin’s new coating stacks
seven of these layers, one on top of the other, in such a way
that each layer enhances the antireflective properties of the
layer below it. These additional layers also help to “bend” the
flow of sunlight to an angle that augments the coating’s
antireflective properties. This means that each layer not only
transmits sunlight, it also helps to capture any light that may
have otherwise been reflected off of the layers below
it.

The seven layers, each with a height of 50 nanometers to 100
nanometers, are made up of silicon dioxide and titanium dioxide
nanorods positioned at an oblique angle — each layer looks and
functions similar to a dense forest where sunlight is
“captured” between the trees. The nanorods were attached to a
silicon substrate via chemical vapor disposition, and Lin said
the new coating can be affixed to nearly any photovoltaic
materials for use in solar cells, including III-V
multi-junction and cadmium telluride.

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Founded in 1824, Rensselaer Polytechnic Institute is America’s first technological research university. Rensselaer encompasses five schools, 32 research centers, more than 145 academic programs, and a dynamic community made up of more than 7,900 students and more than 100,000 living alumni. Rensselaer faculty and alumni include more than 145 National Academy members, six members of the National Inventors Hall of Fame, six National Medal of Technology winners, five National Medal of Science winners, and a Nobel Prize winner in Physics. With nearly 200 years of experience advancing scientific and technological knowledge, Rensselaer remains focused on addressing global challenges with a spirit of ingenuity and collaboration.